The G614 has thin tread by design 12/32 new, for less friction. Be sure to look up the specs on that tire before you discard them. I have provided the link to the specs for that tire. And the 614 is a "retreadable" tire as because it is on a PREMIUM carcass. All you have to do is find a good year authorized retread center, most major cities have one or more. You own the carcass of a fine tire, you might as well put some more rubber on it. If you have concerns about industrial tire retreads do a youtube search and learn how they do it, you might come away with a different feeling these are not your grandpas automotive retreads.
http://www.goodyeartrucktires.com/tires/details.aspx?prodline=160007
Youtube video for a modern retread facility. These facilities will retread your tire and give you back your same tire.
https://www.youtube.com/watch?v=M2_NYEvNrw0
I have never done this but I know people who have and tehy have had good service from the tire, your 614 carcass's have probably never been damaged and are more than suitable for retread.
But most importantly you need to take a measurement with a cheap tread depth gauge and see how much of the original 12/32 tread you have used and I bet that you have not used that much in 20,000 miles. Your tires are OTR tires and have much more life than 20k.
Just trying to save you some money and prevent you from discarding your premium tires for something less. If you gotta buy new, get you another set of 614's
To answer your question, it is my opinion that you should never run your rig faster than 65. The 614 is a 75 mph tire.
If you were to travel across Texas, New Mexico, Arizona, and Nevada, you could see ambients at the road surface of 140 degrees F. The desert floor is very hot. The 614 is handy here in that its thin tread design helps reduce friction so internal heat does not add to the problem.
On a recent trip my G114 were running about 110 but it was not too terribly hot, the ambient was about 93. I really do not know how accurate a TPMS temperature reading is. I just use it as a relative reading but I'm sure the pressures are accurate.
Good luck
I have been paying particular attention to tire issues, and I need some additional information. What speeds do you normally travel, ambient temperature and normal trailer weight. If you could supply this information, I would be most appreciative. Thanks
BTY I have G614's that have near 20k miles that are getting less tread and I will be buying replacements next year-- that is why I am interesting experiences and data from the folks. Price is no object for me, but safety is...